Radio frequency interference (RFI) generated by outboard motors employing capacitor discharge (CD) ignition systems has been a long standing problem, and has caused such motors to be banned from use on certain lakes in Europe.
The environment in which a spark plug must operate in a high speed high compression two-cycle engine such as an outboard motor, motorcycle, or snowmobile, is vastly different from the environment within a four-cycle engine. The center electrode of an automobile type spark plug would overheat and effectively function as a glow plug in such engines and cause damaging preignition. In order to overcome this difficulty annular gap spark plugs which inherently operate at a much lower temperature are now almost universally employed. Unfortunately, the annular gap spark plug runs so cold that it has a tendency to foul, particularly when employed with the conventional inductive automobile type ignition system. Consequently, CD ignition systems are now almost universally employed on high power outboards and their use is gradually being expanded throughout the two-cycle engine field.
CD ignition systems are characterized by their speed of discharge, high voltage and high current across a larger spark gap. Typically, a CD ignition system will build up to 20 to 30 kv across the spark gap in 2 to 3 microseconds, compared to the inductive automotive type system which typically builds up to 10 to 15 kv in approximately 25 to 50 microseconds. The typical CD ignition coil high voltage secondary winding has only 1/10 to 1/5 the turns, and only 1/20 to 1/10 the ohmic resistance of the inductive automotive type secondary winding. The use of a larger spark gap further aids in combatting fouling.
As a result of the aforementioned characteristics of the CD ignition system, the electrical impulse generated upon the breakdown of its annular gap spark plug is significantly more powerful and has an effect on a wider spectrum of frequencies than similar impulses previously encountered. It is this oscillatory electrical impulse with its very high frequency, high voltage excursions in the wiring of the ignition circuit which is the source of RFI attributable to the plug itself.
In automotive inductive type ignition systems the traditional approach to the suppression of RFI has been to insert resistance in series with the secondary of the ignition circuit typically with values of 10,000 ohms or more. However, we have found that the use of added resistance in the secondary of a CD ignition circuit visibly diminishes the brilliant intensity of the spark, and measurably diminishes its duration. Such a resistance slows the discharge of energy through the spark gap and dissipates some of the potential spark energy in the form of useless heat. Further, it may be conclusively stated that insertion of a resistance in excess of approximately 1000 ohms in the body of the spark plug in an attempt to suppress RFI from a CD ignition system will adversely effect the performance of the engine and even this low value of resistance may cause a slight roughness at idle speeds. Therefore, the application of prior art teachings with respect to suppression of RFI through the use of resistors is impractical where CD ignition systems are employed.
It is an objective of the invention to overcome the aforementioned problems by providing a spark plug having a low resistance low inductance suppression element embodied therein, which spark plug will effectively suppress RFI generated by its spark gap; and it is a specific further objective to provide a spark plug which will accomplish the foregoing without degradation in the performance of the engine or ignition system in which it is employed.
It is another objective of the invention to provide a spark plug construction with the aforementioned qualities which is compatible with present methods and equipment for spark plug manufacture, and it is a further objective of the invention to provide a tamper proof suppression device which is in and of itself substantially effective in eliminating RFI from engines which employ CD ignition systems having no significant sources other than the gap discharges of the spark plugs themselves.